Rich nations as a group are unlikely to reach the deep 2020 cuts in greenhouse gas emissions urged by developing nations as part of a new U.N. climate treaty, the top U.S. climate envoy said on Friday.

China, India and other developing nations say the rich must do most to fight global warming to encourage developing countries to sign up for more action as part of a new U.N. climate pact due to be agreed in Copenhagen in December.

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"We...have been engaged in conversations with our European friends about how you might express an aggregate kind of goal," U.S. Special Envoy for Climate Change Todd Stern said in a telephone briefing before a round of U.N. talks in Bonn from June 1-12.

"I don't think you are going to see a 25-to-40 percent aggregate number" for cuts by rich nations below 1990 levels by 2020, he said, adding: "It's possible when you add everything up that you won't be that far away from it."